Insulating circuits having high noise immunity such as insulating amplifiers are used in environments such as electric vehicles and apparatuses for factory automation where high power electrical devices coexist with highly sensitive electronic devices. Optically coupled insulating devices such as photocouplers that optically transmit signals have excellent noise immunity because complete electrical insulation is provided between inputs and outputs.
In optically coupled insulating devices, analog signal data can be transmitted with high precision to circumvent nonlinearity of photocouplers and the like by converting detected analog signals into digital signals before being transmitted optically. Further, optically coupled insulating devices have been proposed in which encoded digital signals and clock signals are transmitted by photocouplers and the like in order to reduce the number of transmission lines (for example, refer to JP-A 2006-303663 (Kokai)).
With regard to optical connection of optically coupled insulating devices, verification of normal transmission is necessary during the specified guarantee period even if temperature change and aging degradation of a light emitting element such as a light emitting diode (LED) occur. Simplified testing methods are preferable in a product testing process for the optically coupled insulating devices.